A known problem in semiconductor manufacturing is the lack of uniformity in the thickness of the layer of isolation material, that is grown or deposited between active areas of a semiconductor substrate, after etch back for planarizing the surface of the isolation material. The term "active area" is commonly used to describe that portion of the semiconductor substrate on which components are built, such as transistors, capacitors and resistors. As is well known in the art, in order to prevent crosstalk between active areas on a substrate, an electrical isolation material, commonly as a refill material, is placed between the active regions. In a silicon-on-insulator (SOI) device, one common method for making space for the refill material is to etch the silicon material above the buried insulator layer to form islands of active material commonly known as mesas, by methods well known in the art. This space is then filled (refilled) with the isolation material. Common refill materials include silicon dioxide and TEOS.
FIG. 1A shows a SOI semiconductor device 100 both before and after the etch back process. The device 100 is built upon a silicon substrate 102 having a buried insulator layer 104. The silicon material above the buried insulator material has already been etched away to form a mesa having a active area 106. Active layer 106 is covered by an isolation layer 108 which is typically a pad oxide of silicon dioxide. On top of pad oxide 108 is an isolation layer 110 which is typically silicon nitride. The resulting structure is then covered with refill layer 112 which may be silicon dioxide or TEOS, for example. As can clearly be seen by FIG. 1A, refill layer 112 does not have a flat topography and the thickness is significantly higher above active area 106.
As is well known in the art, it is desirable to have a flat or planarized surface for the refill layer 112 in order that subsequent layers of the semiconductor device be built on a flat surface. In order to achieve this planarized effect, one technique that is commonly used is to etch back the refill layer 112 until it reaches the isolation layer 110. Dashed lines 114, 116 and 118 represent the refill layer 112 after the etch back process. Line 114 represents the ideal case in which the refill material 112 it etched back until the material just reaches the silicon nitride layer 110. Notice that the resulting layer provides significant refill material on either side of the active area 106 but that this refill material tapers off rapidly as the distance from the side of the active area increases. Thus, in areas where there are no active areas, only a thin layer of refill material remains. In order to make sure that the etch back process has been completed, that is that no refill material remains on the silicon nitride layer, it is common to over-etch the refill material, as shown by dashed line 116. This leaves adequate refill material on the sides of the active area 106 but the material still tapers off away from the active areas in the same manner as shown by dashed line 114. In this case, however, because the material is etched to a greater degree, the etching process will etch away part of the buried insulator region 104 in areas away from active areas of the substrate. This occurs because the refill material is one form of silicon dioxide and the buried insulator layer is typically made of silicon dioxide, so that both oxides will be etched by the same etchant. Process parameters may vary from one operation to the next and dashed line 118 shows the result of the etch back process running too long. Sufficient refill material still remains on either side of active area 106 since the material is above the top of the active area. However, in regions away from the active area, the buried oxide 104 has been significantly etched thereby leaving little or no insulating layer in this region. This results in increased polycrystalline silicon layer to substrate capacitance and crosstalk between the polycrystalline silicon lines.
FIG. 1B shows a SEM photograph of an SOI mesa isolation process using TEOS as the refill material. In this photograph the device has a 0.3 micrometer line and a 0.9 micrometer spacing. As can be seen, the active area has only small refill areas on either side of the mesa that provides only sidewall insulation. The silicon nitride layer above the active areas and the refill areas is there in order to protect the devices on the substrate when they are cleaved for taking the photograph. Thus the silicon nitride layer will not be present in the device. Looking at the bottom of the silicon nitride layer, it can be seen that there is no refill material in the spaces between devices and that the buried insulator layer has been significantly etched away leaving little or no isolation material to prevent crosstalk between devices.
Accordingly, there is a need for a low-cost and effective method for reducing this non-uniformity in the refill layer.